Beyond Silicon

For decades, electronic devices have been getting smaller, and smaller, and smaller. It’s now possible—even routine—to place millions of transistors on a single silicon chip.

But transistors based on semiconductors can only get so small. “At the rate the current technology is progressing, in a decade or two, they won’t be able to get any smaller,” said Michigan Tech physicist Yoke Khin Yap. “Also, semiconductors have another disadvantage: they waste a lot of energy in the form of heat.”

Scientists have experimented with different materials and designs for transistors to address these problems, always using semiconductors like silicon. Back in 2007, Yap wanted to try something different that might open the door to a new age of electronics.

“The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top,” he said. “We chose a nanoscale insulator, boron nitride nanotubes, or BNNTs, for the substrate.” For the metal, they used quantum dots of gold as small as three nanometers across and placed them on the BNNTs.

With scientists at Oak Ridge National Laboratory they fired up electrodes on both ends of the quantum dot–nanotube assembly at room temperature, and something interesting happened. Electrons jumped very precisely from gold dot to gold dot, a phenomenon known as quantum tunneling.

“Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river,” said Yap. “The electrons hopped between the gold stepping stones. The stones are so small you can only get one electron on the stone at a time. Every electron is passing the same way, so the device is always stable.”

They had made a transistor without a semiconductor. When sufficient voltage was applied, it switched to a conducting state. When the voltage was low or turned off, it reverted to its natural state as an insulator.

Furthermore, no electrons from the gold dots escaped into the insulating BNNTs, thus keeping the tunneling channel cool. In contrast, silicon is subject to such leakage, which wastes energy in electronic devices and generates a lot of heat.

Other people have made transistors that exploit quantum tunneling, says Michigan Tech physicist John Jaszczak, who has developed the theoretical framework for Yap’s experimental research. However, those tunneling devices have only worked in conditions that would discourage the typical cellphone user.

“They only operate at liquid-helium temperatures,” said Jaszczak.

The secret to Yap’s gold-and-nanotube device is its submicroscopic size: one micron long and about 20 nanometers wide. “The gold islands have to be on the order of nanometers across to control the electrons at room temperature,” Jaszczak said. “If they are too big, too many electrons can flow.” In this case, smaller is truly better: “Working with nanotubes and quantum dots gets you to the scale you want for electronic devices.”